Depends on your budget.
If you have a few thousand dollars to spend, go with a few Tunze's They're the best powerheads you could possibly buy.
If you are on a budget and you still need really good flow, get some Hydo Koralia 5's. One in the back and one in the front should work. If you have any extra problems with flow, you should get a couple of rio's.
For a 240 gallon tank, you should really have gotten it drilled for a closed loop. Tanks that big can't go without a closed loop - it's way more efficiant then powerheads, and it looks better because there's no cords coming out of your tank.
Anyway, just my 0.02